(a) Field of the Invention
The present invention relates to a lens system for optical recording type disks which is small in size and light in weight.
(b) Description of the Prior Art
For a lens system to be used for optical recording type disks, it is necessary to make the resolving power 1.mu. or less because the lens system is used for reading out the very small signals recorded with high density on the disk. To obtain a lens system with the resolving power of 1.mu. or less as mentioned in the above, spherical aberration and sine condition should be made as small as possible and, moreover, astigmatism should be also made small. As the automatic tracking method for optical recording type disks, two methods are known, i.e., a method to vibrate the light beam by using a galvanomirror and a method to move the lens system itself in parallel with the disk surface. The lens system for optical recording type disks according to the present invention is to be used for a device for optical recording type disks employing the latter tracking method. Therefore, the lens system should be light in weight and paraxial aberrations thereof should be corrected favourably.
Furthermore, if the lens system for optical recording type disks comes into contact with the disk, the disk and lens system are broken. To prevent the above, the lens system for optical recording type disks should have a long working distance. Besides, for the lens system for optical recording type disks, it is preferable to make the number of lenses constituting the lens system as small as possible.